There is a good reason why agents ask for the first three chapters of your novel. The same reasoning can be applied to the first page or couple of pages of a short story, the opening act of a play, the first twenty minutes of a full-length movie.
I have talked before about the need to know what your protagonist wants and this alone can define the shape of your story; but by the end of the opening section, we need to know who, why, when and a hint of how. The opening ends with the first inciting incident; we need to know who the protagonist is (without those plot-stopping blocks of exposition), we need to have a sense of why he/she is where they are (in life, what’s missing perhaps?), we need context for when (place, time etc.) this thing happens that creates the driving conflict of the story; and while you will have a period of floundering where the character decides what to do (the reluctant hero) we also need a sense of how this might be put into action.
While the action has not have completely started by this point, all the essential ingredients should be there so the reader knows who to root for, appreciates the significance of the incident that will shape the story: to that character and have a sense of what this story is, what kind of story to expect … is it quest, a find a missing child story, a crime story, a literary character-driven story, a love story etc. and they must be rooting for the protagonist. To root you must have hooked the reader and developed empathy. If you are asking your reader to invest in this story, then they have to care enough.
So have a look at some of the openings of books you love, books that you don’t think work and your own work if you write. Do you do all of these things? It’s about balance. Don’t overload the opening, show the reader enough to intrigue while doing all of these things, and they will stay on the journey with you.
And on that note, as I return to my allotment (in a literal sense), I bid you all a fantastic weekend, whatever you do!